From the past, condensation curable silicone resin compositions are used as encapsulant for semiconductor chips such as LED because of excellent heat resistance and light resistance. For example, Patent Document 1 describes a condensation curable silicone resin composition which is resistant to sulfide and effective for the protection of bottom silver surface in LED packages. Patent Document 2 describes a condensation curable silicone resin composition having improved adhesion to adherends.
Although condensation curable silicone resin compositions are widely used as the semiconductor encapsulant, their properties are still below the satisfactory level. In particular, LED encapsulants are exposed to not only internal stresses by temperature changes associated with on/off operation of optical semiconductor devices, but also external stresses by changes of ambient temperature and humidity. For the LED encapsulants, not only heat resistance and light resistance, but also crack resistance are important. However, silicone resins obtained by curing prior art condensation curable silicone resin compositions do not withstand stresses by temperature changes, i.e., have poor crack resistance. In general, a countermeasure taken in silicone resin for the purpose of improving crack resistance is by reducing the hardness of resin to make the resin softer so that the stress may be mitigated. However, once the silicone resin is softened, another problem arises that the resin is deformable in shape and sticky on its surface. Such resin products are difficult to handle. Their gas barrier properties are degraded, and LED protection ability is reduced.
For imparting toughness to a cured silicone resin while maintaining hardness, an attempt to incorporate a silphenylene skeleton into a silicone resin is made in Patent Documents 3 and 4. As compared with the general method of establishing a high hardness by increasing the crosslinking density of silicone resin, this method establishes a high hardness by incorporating a silphenylene skeleton into a silicone resin to restrain motion of the polymer chain. The resulting resin has rigidity and hardness. While the method of increasing the crosslinking density generally makes the resin brittle, the method of incorporating a silphenylene skeleton is advantageous in that the resin exerts a toughness without embrittlement because the silphenylene skeleton is linear. However, the method of incorporating a silphenylene monomer into a silicone resin skeleton as described in Patent Documents 3 and 4, achieves weak binding of the molecular chain and thus fails to form a resin having a high hardness and yet satisfactory stress resistance.